1. Field of the Invention
The present invention relates to a method of manufacturing a metal foil laminated product in which a metal foil is laminated integrally, by utilizing a bonding layer, on a wiring layer provided with a wiring pattern or on a metal layer or an insulating layer which has no pattern and a method of manufacturing a wiring board, and more particularly to a useful technique for manufacturing a multilayer wiring board having a conductive connecting structure between wiring layers.
2. Description of the Related Art
In the steps of forming a core substrate, an insulating layer or the like of a printed wiring board to be used in electronic equipment or the like, conventionally, there has been used a prepreg in which a glass fiber fabric or a polymer nonwoven fabric is impregnated with a thermosetting resin and is half cured. The prepreg is thermally pressed together with copper foils provided on both surfaces, for example, so that a double-sided copper foils laminated plate can be obtained, and a wiring pattern is formed on the copper foil and can be thus used as the core substrate of a multilayer wiring board.
In recent years, the wiring board has a finer wiring pattern because of the high integration of a wiring, and furthermore, a multilayer structure is achieved and each layer is thinned. For this reason, in the case in which a glass fiber fabric impregnated with an epoxy resin is used for the prepreg, there is a problem in that drilling is carried out with difficulty due to a glass fiber when laser via processing is to be carried out. Therefore, there have been proposed various prepregs using, for a reinforcing phase, a polymer nonwoven fabric on which the laser via processing can be carried out more easily.
However, if an insulating layer is to be provided on a wiring layer having a wiring pattern formed thereon by utilizing a prepreg using a polymeric nonwoven fabric for a reinforcing phase, a wiring layer formed as an upper layer is not flat due to the irregularity of the wiring pattern or nonwoven fabric itself. Consequently, there is a problem in that a step of providing an upper layer which is to be subsequently carried out is impeded. In a multilayer wiring board in which each layer has a thickness reduced, particularly, there has been employed a method in which a thermosetting resin is applied in place of the provision of the prepreg or a thermosetting resin sheet is provided to form an insulating layer. In these methods, however, the reinforcing phase is not present. Therefore, it is hard to control a gap for forming the insulating layer. Thus, such a method is not advantageous to mass production.
On the other hand, there has also been known a prepreg using a porous film comprising aromatic polyamide in place of the polymeric nonwoven fabric (Japanese Laid-Open Patent Publication No. 9-324060). As described in the publication, generally, when a porous film is to be impregnated with a thermosetting resin, a resin is dissolved in a solvent to prepare a varnish, the porous film is coated and impregnated with the varnish, and the solvent is evaporated by heating and is thus half cured.
However, it has been found that the porous film is swollen by the solvent contained in the varnish or the porous film is deformed by a stress generated during the coating if the porous film is thus coated directly with the varnish, and a surface is wrinkled to easily generate irregularities, particularly in the case where a releasing film is particularly provided. As a result, when a conductive paste is to be applied into a via hole by screen printing after laser via processing, for example, there is a problem in that the screen printing cannot be carried out.
Therefore, it is an object of the present invention to provide a method of manufacturing a metal foil laminated product in which a metal foil laminated product having a flat surface can be formed and a gap for laminating a metal foil can easily be controlled so that mass production can be advantageously carried out and a problem is less caused by a solvent during the formation of a bonding layer, and a method of manufacturing a wiring board.
The above-mentioned object can be achieved by the present invention in the following manner.
A method of manufacturing a metal foil laminated product of the present invention comprises the steps of: forming a bonding layer containing a thermosetting resin on a lower wiring layer, a metal layer or an insulating layer; provisionally bonding a porous layer having a releasing film attached thereto onto a surface of said bonding layer; peeling said releasing film from said porous layer; laminating a metal foil on the porous layer obtained after the peeling; and heating and pressurizing the laminated product to transfer said bonding layer to said metal foil and thereby integrating them.
According to the method of manufacturing a metal foil laminated product of the present invention, a reinforcing material of a prepreg is formed by a porous layer. Therefore, a metal foil laminated product having a flat surface can be formed without generating irregularities by a reinforcing fibers, and the thickness of an insulating layer can be reduced by the porous layer so that a density can be increased in a thickness direction. Moreover, the porous layer has a function like a spacer. Therefore, the gap control of the insulating layer can easily be carried out by a pressurizing force, so that mass production can be achieved advantageously. Furthermore, a problem is less caused by a solvent during the formation of a bonding layer (an impregnated phase) as compared with the case in which a resin is previously impregnated in the hole of a porous film.
Another method of manufacturing a metal foil laminated product of the present invention comprises the steps of: forming a bonding layer containing a thermosetting resin on an insulating layer; forming a wiring layer having a pattern on a surface of the bonding layer; provisionaly bonding a porous layer having a releasing film attached thereto onto an exposed surface of said bonding layer via said wiring layer interposed therebetween; providing an opening reaching the wiring layer from said releasing film; filling the opening with a conductive paste; peeling the releasing film from the porous layer; laminating a metal foil on a surface where the peeling has been carried out; and heating and pressurizing the laminated product to transfer said bonding layer to the metal foil and thereby integrating them.
In this case, it is possible to obtain the above-mentioned functions and effects, and furthermore, to form a conductive connecting structure between the wiring layers by a conductive paste. In that case, the thickness of the insulating layer can be reduced, and therefore, the diameter size of an opening formed by a laser processing can be reduced so that a wiring pattern can become finer.
On the other hand, a method of manufacturing a wiring board of the present invention comprises the steps of: manufacturing a metal foil laminated product by the manufacturing method according to the above methods; and providing a pattern on the metal foil of the metal foil laminated product, thereby forming a wiring layer. According to the method of manufacturing a wiring board in accordance with the present invention, a metal foil laminated product having a flat surface can be formed by the above-mentioned functions and effects. In addition, a gap for laminating a metal foil can easily be controlled. Therefore, mass production can be achieved advantageously and a bonding layer can also be formed readily.